UNIQUE constraint failed on adding a new model field

HI . when i want to add a new field called "slug" to my "Post" model, the migrate command will raise UNIQUE constraint failed: new__chat_post.slug

after that i remove that field from my model but the problem still exists. why?? and how to resolve this problem without deleting my whole table datas. thanks ..

the model:

class Post(models.Model):    

    title = models.TextField(max_length=100)

    context = models.TextField(max_length=255)

    creation_date  = models.DateTimeField(default = timezone.now)

    author = models.ForeignKey(settings.AUTH_USER_MODEL,on_delete = models.CASCADE)

    slug = models.SlugField(default=["title"],unique=True)

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